Here is the Plugin;

You can see what code the plugin has;

<?php /*
Plugin Name: Nice Search
Version: 0.2
Plugin URI: http://txfx.net/code/wordpress/nice-search/
Description: Redirects ?s=query searches to /search/query, and converts %20 to +
Author: Mark Jaquith
Author URI: http://txfx.net/
*/

function txfx_nice_search() {
	if ( is_search() && strpos($_SERVER['REQUEST_URI'], '/wp-admin/') === false && strpos($_SERVER['REQUEST_URI'], '/search/') === false ) {
		wp_redirect(get_bloginfo('home') . '/search/' . str_replace(' ', '+', str_replace('%20', '+', get_query_var('s'))));
		exit();
	}
}

add_action('template_redirect', 'txfx_nice_search');
?>

That’s All!
You can download it from here:
Note: There is a file embedded within this post, please visit this post to download the file.

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